1. Field of the Invention
This invention relates to intracranial access. Generally, this invention relates to using imagery to establish a trajectory and distance for introduction of a device to a site in the brain and, more particularly, to directed placement of a device to the cerebral ventricles.
2. Description of Related Art
Features in and around the brain (the intracranial contents) may be accessed for diagnosis or treatment by way of a hole drilled through the skull. Accessing the intracranial contents may include introducing a device, such as a catheter or needle, through the hole to a particular site.
It may be necessary to place the device with some degree of accuracy. Particularly, where it is desired to place the tip of a device such as a catheter or a cerebrospinal fluid shunt or a needle into a cerebral ventricle, accurate placement of the tip of the device is necessary.
Accurate intracranial placement of devices is challenging. Conventionally, the placement at the desired position (the target) may be approximated by making the hole in the skull at one of the known landmark locations on the skull, and then introducing the device through the hole in a direction and to a depth that the surgeon estimates will locate the device tip at the target. Stated another way, the conventional approach, representing a current standard of practice, is essentially “blind”, entailing some degree of guesswork, and as a result the placement is not always satisfactorily accurate. Misplaced devices may be ineffective, and may result in harm to the patient. Accordingly, it may be necessary to remove and reintroduce an inaccurately placed device one or more additional times, until placement is deemed satisfactory. Repeated placement efforts increase risks of injury or trauma to the patient, such as bleeding or damage to brain tissue or infection.
Ventricular catheters and ventricular shunts are typically left in place for some time following emplacement, with the distal tip at the target and the proximal end outside the cranium. A ventricular catheter may serve as a drain, for example, to control flow of fluid from the ventricles; or, a catheter may serve as a conduit for introduction of a diagnostic or therapeutic substance to the target. The proximal end of the catheter may be connected to a reservoir, from which fluid (such as cerebrospinal fluid) may be removed or into which a therapeutic or diagnostic substance may be introduced. It is preferred to limit movement or play in the location of the device at the hole, to prevent displacement from the target after placement. Accordingly, where the device is to be left in place, it is desirable to employ a small hole in the skull, and typically the hole in the skull for a device that is left in place for some time has a diameter only a few millimeters larger than the diameter of the device.
Various stereotactic guidance systems have been proposed to improve targeting of intracranial sites; these generally must be deployed in an operating theater.
In one approach to directed placement of a catheter, the catheter itself is provided with ultrasound capability. Schultz-Stubner U.S. Patent Publication No. 2007/0083100, for example, describes an ultrasound probe associated with the distal end of a ventriculostomy catheter, operable to provide ultrasound imaging during advancement of the catheter or when the catheter is positioned at a desired location in a cerebral ventricle. Gilbert U.S. Pat. No. 5,690,117 describes an intracranial catheter having a stylet provided with fiberoptics and an ultrasound transducer. Ultrasound probes typically produce a beam that is the same size as the probe cross section, or only slightly larger, and the miniaturized ultrasound probes in these devices are too small to provide an interpretable image that would be deep enough and broad enough to visualize a target and distinguish it from surrounding tissues. Moreover, the imaging parts of these devices are very costly, and the devices are not likely to be reusable, so they are too expensive to be accepted.
Boner et al. U.S. Pat. No. 4,681,103 describes a stereotactic guide for obtaining needle biopsies from the brain. It includes a mounting assembly that is screwed into a burr hole in the patient's skull, and a swivel ball disposed in the assembly, and a locking ring disposed over the swivel ball, which can be reversibly tightened to fix the swivel ball in place or loosened to allow it to swivel. The swivel ball constitutes a socket that receives an intraoperative ultrasound probe, which can be removed and replaced with a needle guide. The “probe is as close to the brain as possible”, and is shown as projecting into the hole until it is at or below the level of the dura (that is, at or below the inner table of the skull). Accordingly, the burr hole must be vary large, to accommodate these features. The biopsy needle is inserted through the needle guide, the biopsy is taken, and the needle is removed.